Magnesium sulfate prophylaxis in preeclampsia: Lessons learned from recent trials

In the US, the routine use of magnesium sulfate for seizure prophylaxis in women with preeclampsia is an ingrained obstetric practice. During the past decade, several observational studies and randomized trials have described the use of various regimens of magnesium sulfate to prevent or reduce the rate of seizures and complications in women with preeclampsia. There are only 2 double-blind, placebo-controlled trials evaluating the use of magnesium sulfate in mild preeclampsia. There were no instances of eclampsia among 181 women assigned to placebo, and there were no differences in the percentage of women who progressed to severe preeclampsia (12.5% in magnesium group vs 13.8% in the placebo group, relative risk [RR] 0.90; 95% CI 0.52-1.54). However, the number of women enrolled in these trials is too limited to draw any valid conclusions. There are 4 randomized controlled trials that compare the use of no magnesium sulfate, or a placebo vs magnesium sulfate, to prevent convulsions in patients with severe preeclampsia. The rate of eclampsia was 0.6% among 6343 patients assigned to magnesium sulfate vs 2.0 % among 6330 patients assigned to a placebo or control (RR 0.39; 95% CI 0.28-0.55). However, the reduction in the rate of eclampsia was not associated with a significant benefit in either maternal or perinatal outcome. In addition, there was a higher rate of maternal respiratory depression among those assigned magnesium sulfate (RR 2.06; 95% CI 1.33-3.18). The evidence to date confirms the efficacy of magnesium sulfate in reduction of seizures in women with eclampsia and severe preeclampsia; however, this benefit does not affect overall maternal and perinatal mortality and morbidities. The evidence regarding the benefit-to-risk ratio of magnesium sulfate prophylaxis in mild preeclampsia remains uncertain, and does not justify its routine use for that purpose.